1. Field of the Invention
This invention generally pertains to spark gaps and more particularly relates to triggered spark gaps wherein the trigger electrode, or a substantial portion of it, is disposed in the region between the main electrodes.
2. Description of Prior Art
Triggered spark gaps were devised to make insulating gaps in electrical circuits, normally high voltage circuits, electrically conductive. Additionally, triggered spark gaps have been found to switch from the nonconducting state to the conducting state extremely rapidly and with a timing accuracy, or jitter, of a few nanoseconds. As a result of this and other features triggered spark gaps have found substantial acceptance and circuits such as the Marx generator have been developed which take advantage of the unique characteristics of these devices.
Generally speaking, the two sides of a gap in an electrical circuit are connected to opposite electrodes of the spark gap. The spark gap thus formed may then be closed by applying a high voltage pulse, or step, to a third electrode known as the trigger electrode. This trigger electrode may lie partly inside one main electrode, or alternatively, may be disposed between the main electrodes. Spark gaps of this construction may be triggered and closed with delays of 10 nanoseconds and with an accuracy, or jitter, of less than 1 nanosecond.
Before switching, the voltage of the trigger electrode is held at a potential between the voltages of the main electrodes. The spark gap may then be switched by applying a voltage pulse, or step, to the trigger electrode. When this is done, the voltage difference between the trigger electrode and first of the main electrodes is decreased whilst the voltage difference between the trigger electrode and the second of the main electrodes is increased. If this latter voltage difference is sufficiently large, the gap between the trigger electrode and the second main electrode will be crossed by an arc quickly bringing the voltage on the trigger electrode to that of the second main electrode. At this point the voltage difference that was originally applied between the main electrodes is applied between the trigger electrode and the first main electrode. If this voltage is sufficiently large, this gap is closed by an arc thus completing the switching action. The breakdown of these two gaps, that is between the trigger electrode and each of the two main electrodes, is partly due to the increased average electric field and partly due to the distortion of the electric field caused by the change of voltage of the trigger electrode. This switching action is usually helped by the presence of sharp edges on the trigger electrode which cause a localized enhancement of the electric field above the strength of the switching medium in their immediate vicinity. The localized field enhancement so caused acts additively with the change of voltage of the trigger electrode during the application of the voltage pulse thereto to produce a field distortion of greater magnitude than the voltage change alone could produce and thereby facilitates accurate switching.
In conventional triggered spark gaps, the cross sectional dimension of the trigger electrode extending through the gap parallel to the direction of current flow, hereinafter thickness, is less than or equal to the cross sectional dimension perpendicular thereto, hereinafter width, in order to minimize the field distortion caused by the trigger electrode and to maximize the strength of the gap. The action of the arcs, however, causes erosion of the trigger electrode primarily at the corners where the electric field, and thus arcing, is maximum. This blunting of the corners in turn reduces the maximum electric field and hence the triggering ability of the trigger electrode. The magnitude of this erosion effect is directly related to the level of current passed by the gap, the higher the current passed the worse the erosion. For this reason, some workers in the art have used pre-blunted and/or rounded trigger electrodes so that erosion effects will cause smaller alterations in performance over the life of the gap. This solution to the erosion problem while beneficial in certain particular instances entails a significant sacrifice in usuable triggering range throughout the life of the switch and is consequently of restricted application.
The switching medium used in a triggered spark gap can be a liquid, a gas or a mixture of gases. Conventional spark gap switches generally include a housing, or container, whereby the switching medium is retained, and such housings may also serve as a frame to maintain the relative positions of the electrodes. In the event that air is to serve as the switching medium, however, no container or housing is required, but a frame to maintain relative electrode positions is required. Similarly, a frame, on which the electrodes are mounted, is required where one or more gaps are part of a device which is hermitically sealed in a container.